Fuel cell with enhanced mass transfer characteristics

ABSTRACT

Disclosed is a fuel cell with enhanced mass transfer characteristics in which a highly hydrophobic porous medium, which is prepared by forming a micro-nano dual structure in which nanometer-scale protrusions with a high aspect ratio are formed on the surface of a porous medium with a micrometer-scale roughness by plasma etching and then by depositing a hydrophobic thin film thereon, is used as a gas diffusion layer, thereby increasing hydrophobicity due to the micro-nano dual structure and the hydrophobic thin film. When this highly hydrophobic porous medium is used as a gas diffusion layer for a fuel cell, it is possible to reduce water flooding by efficiently discharging water produced by an electrochemical reaction of the fuel cell and to improve the performance of the fuel cell by facilitating the supply of reactant gases such as hydrogen and air (oxygen) to a membrane-electrode assembly (MEA).

CROSS-REFERENCE TO RELATED APPLICATION

This application claims under 35 U.S.C. §119(a) the benefit of KoreanPatent Application No. 10-2011-0102843 tiled Oct. 10, 2011, the entirecontents of which are incorporated herein by reference.

BACKGROUND

(a) Technical Field

The present invention relates to a fuel cell and a method formanufacturing the same. More particularly, it relates to a fuel cellwith enhanced mass transfer characteristics and a method formanufacturing the same, which can reduce water flooding by efficientlydischarging water produced by an electrochemical reaction using a highlyhydrophobic gas diffusion layer having a new surface structure and canimprove cell performance in a high power density region and in anabnormal operating condition by facilitating the supply of reactantgases such as hydrogen and air (oxygen) to a membrane-electrodeassembly.

(b) Background Art

Typically, one of the most attractive fuel cells for a vehicle is apolymer electrolyte membrane fuel cell (PEMFC) manufactured by stackingseveral hundreds of unit cells into a stack. In order to mount the PEMFCin a vehicle for use of transport, the PEMFC should exhibit a high powerperformance of several tens of kW or higher under various operatingconditions and, to this end, should be able to stably operate in a widecurrent density range.

The electrochemical reaction for electricity generation of the PEMFCwill be described below. Hydrogen supplied to an anode as an oxidationelectrode in a membrane electrode assembly (MEA) of the fuel cell isdissociated into hydrogen ions (protons) and electrons. The hydrogenions are transmitted to a cathode as a reduction electrode through apolymer electrolyte membrane, and the electrons are transmitted to thecathode through an external circuit so that electricity is generated bythe flow of electrons. Moreover, at the cathode, the protons, electronsand oxygen molecules react with each other to produce electricity andheat and, at the same time, produce water as a reaction by-product.

The area expressing the electrochemical performance of the fuel cell isgenerally classified into three regions: (i) an “activation loss” regiondue to loss of electrochemical reaction kinetics; (ii) an “ohmic loss”region due to contact resistance at interfaces between respectivecomponents and loss of ionic conduction in the polymer electrolytemembrane; and (iii) a “mass transport or transfer loss” or“concentration loss” region due to the limitations of mass transport ofreactant gases [See R. O Hayre, S. Cha, W. Colella, F. B. Prinz, FuelCell Fundamentals, Ch. 1, John Wiley & Sons, New York (2006), which ishereby incorporated by reference].

When an appropriate amount of water produced during the electrochemicalreaction is present, it preferably serves to maintain the humidity ofthe polymer electrolyte membrane. However, when an excessive amount ofwater produced is not appropriately removed, “flooding” occurs at a highcurrent density, preventing the reactant gases from being efficientlysupplied to the fuel cell and thereby increasing voltage loss [See M. M.Saleh, T. Okajima, M. Hayase, F. Kitamura, T. Ohsaka, J. Power Sources,167, 503 (2007), which is hereby incorporated by reference].

Recently, with the commercialization of the fuel cell, extensiveresearch and development of gas diffusion layers as a key component ofwater management in the fuel cell has continued to progress. A gasdiffusion layer which is typically included in the fuel cell will bedescribed in detail below.

A typical porous medium that constitutes the fuel cell is a gasdiffusion layer (GDL), which is composed of both a microporous layer(MPL) and a macroporous substrate or backing.

At present, commercially available gas diffusion layers have a duallayer structure including a microporous layer (MPL) having a pore sizebelow 1 micrometer when measured by mercury intrusion and a macroporoussubstrate or backing having a pore size of 1 to 300 micrometers [See, X.L. Wang, H. M. Zhang, J. L. Zhang, H. F. Xu, Z. Q. Tian, J. Chen, H. X.Zhong, Y. M. Liang, and B. L. Yi, Electrochimica Acta, 51, 4909 (2006)which is hereby incorporated by reference].

The gas diffusion layer is attached to the outer surface of each ofcatalyst layers for the anode and cathode coated on both surfaces of thepolymer electrolyte membrane in the fuel cell. The gas diffusion layerfunctions to supply reactant gases such as hydrogen and air (oxygen),transmit electrons produced by the electrochemical reaction, anddischarge water produced by the reaction to minimize the floodingphenomenon in the fuel cell [See L. Cindrella, A. M. Kannan, J. F. Lin,K. Saminathan, Y. Ho, C. W. Lin, J. Wertz, J. Power Sources, 194, 146(2009); and X. L. Wang, H. M. Zhang, J. L. Zhang, H. F. Xu, Z. Q. Tian,J. Chen, H. X. Zhong, Y. M. Liang, B. L. Yi, Electrochim. Acta, 51, 4909(2006) which are both hereby incorporated by reference].

Typically, the microporous layer of the gas diffusion layer may beformed by preparing a mixture of carbon black powder such as acetyleneblack carbon, black pearl carbon, etc. and a hydrophobic agent such aspolytetrafluoroethylene (PTFE) or fluorinated ethylene propylene (FEP).The mixture can be coated on one or both sides of the macroporoussubstrate.

The macroporous substrate of the gas diffusion layer is generallycomposed of carbon fiber and a hydrophobic agent such aspolytetrafluoroethylene and fluorinated ethylene propylene [See, C. Limand C. Y. Wang, Electrochim. Acta, 49, 4149 (2004)] and may be broadlyclassified into carbon fiber felt, carbon fiber paper, and carbon fibercloth [S. Escribano, J. Blachot, J. Etheve, A. Morin, R. Mosdale, J.Power Sources, 156, 8 (2006); M. F. Mathias, J. Roth, J. Fleming, and W.Lehnert, Handbook of Fuel Cells-Fundamentals, Technology andApplications, Vol. 3, Ch. 42, John Wiley & Sons (2003) which are bothhereby incorporated by reference].

It is necessary to optimize the structural design of the gas diffusionlayer for the fuel cell such that the gas diffusion layer providesappropriate performance according to its application fields, such astransportation, portable, and residential power generation devices, andthe fuel cell operational conditions. In general, in the formation ofthe gas diffusion layer for a fuel cell vehicle, the carbon fiber feltor carbon fiber paper is preferred to the carbon fiber cloth since thecarbon fiber felt and carbon fiber paper have excellent properties suchas reactant gas supply properties, product water discharge properties,compression properties, and handling properties.

Moreover, the gas diffusion layer has a significant effect on theperformance of the fuel cell according to complex and various structuraldifferences such as the thickness, gas permeability, compressibility,hydrophobicity of microporous layer and macroporous substrate, carbonfiber structure, porosity/pore size distribution, pore tortuosity,electrical resistance, bending stiffness, etc. Especially, it is knownthat there is a significant difference in performance in the masstransport region [See D. H. Ahmed, H. J. Sung, and J. Bae, Int. J.Hydrogen Energy, 33, 3767 (2008); and Y. Wang, C. Y. Wang, and K. S.Chen, Electrochim. Acta, 52, 3965 (2007); and C. J. Bapat and S. T.Thynell, J. Power Sources, 185, 428 (2008); which are herebyincorporated by reference].

In particular, in order to increase the mass transfer characteristicsand maintain high cell performance by effectively removing the waterproduced during the electrochemical reaction of the fuel cell, it isvery important to impart hydrophobicity to the microporous layer and themacroporous substrate by appropriately introducing a hydrophobic agentsuch as polytetrafluoroethylene (PTFE) into them [See S. Park, J. -W.Lee, B. N. Popov, J. Power Sources, 177, 457 (2008); and G. -G. Park. Y.-J. Sohn, T. -H. Yang, Y. -G. Yoon, W. Y. Lee, C. -S. Kim, J. PowerSources, 131, 182 (2004) which are hereby incorporated by reference].

However, a wet chemical process has conventionally been used to imparthydrophobicity, and thus the manufacturing process itself is complicatedand it is difficult to uniformly distribute the hydrophobic agent suchas PTTE on the gas diffusion layer.

Moreover, according to the conventional process for manufacturing thegas diffusion layer, it is difficult to further impart highhydrophobicity or super-hydrophobicity corresponding to a contact angle(static constant angle) of 150° or higher to a porous medium which havealready been subjected to waterproof treatment.

In conventional studies, there are various attempts to imparthydrophilicity to the surface of the porous medium using various plasmaprocesses such as oxygen, nitrogen, ammonia, silane (SiH₄),organometallics, etc., which, however, are different from the object ofthe present invention to impart high hydrophobicity to the porousmedium.

In addition, there are attempts to employ plasma surface treatmenttechniques during the formation of the electrodes of the MEA, which,however, relate to a process for forming a catalyst layer comprisingcatalyst and binder. That is, these methods are to chemically form ahydrophilic or hydrophobic surface by modifying the surface of thecatalyst layer using plasma techniques, and with these methods, it isvery difficult to form high hydrophobicity on the surface of the porousmedium.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the invention and thefore it may contain information that does not form the prior art that isalready known in this country to a person of ordinary skill in the art.

SUMMARY OF THE DISCLOSURE

The present invention provides a fuel cell with enhanced mass transfercharacteristics and a method for manufacturing the same, which canreduce water flooding by efficiently discharging water produced by anelectrochemical reaction using a highly hydrophobic gas diffusion layerhaving a new surface structure and can improve cell performance in ahigh power density region and in an abnormal operating condition byfacilitating the supply of reactant gases such as hydrogen and air(oxygen) to a membrane-electrode assembly.

In one aspect, the present invention provides a fuel cell with enhancedmass transfer characteristics, the fuel cell comprising amembrane-electrode assembly, a gas diffusion layer as a porous medium,and a separator having flow fields of reactant gases, which are stackedto form a unit cell. The gas diffusion layer may include a micro-nanodual structure, in which nanometer-scale protrusions or collapsed poresare formed on the surface of the porous medium with a micrometer-scalesurface roughness, and a hydrophobic thin film deposited on the surfaceof the micro-nano dual structure.

In another aspect, the present invention provides a method ofmanufacturing a fuel cell with enhanced mass transfer characteristics.In particular, this method provides a porous medium with amicrometer-scale surface roughness; forming a micro-nano dual structureon the surface of the porous medium by forming nanometer-scaleprotrusions or collapsed pores by plasma etching; depositing ahydrophobic thin film on the surface of the micro-nano dual structure;and forming a fuel cell together with a-membrane-electrode assembly anda separator using the porous medium including the micro-nano dualstructure and the hydrophobic thin film as a gas diffusion layer.

Other aspects and exemplary embodiments of the invention are discussedinfra.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will now bedescribed in detail with reference to certain exemplary embodimentsthereof illustrated the accompanying drawings which are givenhereinbelow by way of illustration only, and thus are not limitative ofthe present invention, and wherein:

FIG. 1 is a schematic diagram illustrating a process of forming amicro-nano complex (dual) structure by performing plasma etching on thesurfaces of a microporous layer and a macroporous substrate andimparting high hydrophobicity by performing hydrophobic treatment on theresulting surfaces;

FIG. 2 is scanning electron microscope (SEM) images of the surfaces ofgas diffusion layers (i.e., microporous layers and macroporoussubstrates) before and after oxygen plasma etching for forming amicro-nano dual structure on the surfaces of gas diffusion layers andafter “oxygen plasma etching+hydrophobic carbon thin film (HMDSO)coating”;

FIG. 3 is a graph illustrating changes in contact angle of gas diffusionlayers according to the time of oxygen plasma etching for forming amicro-nano dual structure on the surfaces of a microporous layer and amacroporous substrate and hydrophobic carbon thin film (HMDSO) coatingconditions;

FIG. 4 is a graph illustrating changes in electrical resistance of gasdiffusion layers according to the time of oxygen plasma etching forforming a micro-nano dual structure and hydrophobic carbon thin film(HMDSO) coating conditions during a first compression;

FIG. 5 is a graph illustrating changes in electrical resistance of gasdiffusion layers according to the time of oxygen plasma etching forforming a micro-nano dual structure and hydrophobic carbon thin film(HMDSO) coating conditions during a second compression;

FIG. 6 is a graph illustrating changes in fuel cell performance beforeand after oxygen plasma etching for forming a micro-nano dual structureon gas diffusion layers and hydrophobic carbon thin film (HMDSO) coatingin a condition where the stoichiometric ratio (anode/cathode) is1.5/2.0; and

FIG. 7 is a graph illustrating changes in fuel cell performance beforeand after oxygen plasma etching for forming a micro-nano dual structureon gas diffusion layers and hydrophobic carbon thin film (HMDSO) coatingin a condition where the stoichiometric ratio (anode/cathode) is1.1/1.5.

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variouspreferred features illustrative of the basic principles of theinvention. The specific design features of the present invention asdisclosed herein, including, for example, specific dimensions,orientations, locations, and shapes will be determined in part by theparticular intended application and use environment.

In the figures, reference numbers refer to the same or equivalent partsof the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION

Hereinafter reference will now be made in detail to various embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings and described below. While the invention will bedescribed in conjunction with exemplary embodiments, it will beunderstood that present description is not intended to limit theinvention to those exemplary embodiments. On the contrary, the inventionis intended to cover not only the exemplary embodiments, but alsovarious alternatives, modifications, equivalents and other embodiments,which may be included within the spirit and scope of the invention asdefined by the appended claims.

It is understood that the term “vehicle” or “vehicular” or other similarterm as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, watercraft including a variety ofboats and ships, aircraft, and the like, and includes hybrid vehicles,electric vehicles, plug-in hybrid electric vehicles, hydrogen-poweredvehicles and other alternative fuel vehicles (e.g., fuels derived fromresources other than petroleum). As referred to herein, a hybrid vehicleis a vehicle that has two or more sources of power, for example bothgasoline-powered and electric-powered vehicles.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. As used herein, the term “and/or”includes any and all combinations of one or more of the associatedlisted items.

The above and other features of the invention are discussed infra.

The present invention provides a fuel cell with enhanced mass transfercharacteristics and a method for manufacturing the same, in which ahighly hydrophobic gas diffusion layer having a porous surface withenhanced hydrophobicity is used. In particular, the highly hydrophobicgas diffusion layer of the present invention has a surface with amicro-nano dual structure in which nanometer-scale protrusions orcollapsed pores are formed on the surface of a porous medium with amicrometer-scale roughness (a macroporous substrate, which will bedescribed later, has a micrometer-scale surface roughness) and, at thesame time, a hydrophobic thin film is deposited on the surface of themicro-nano dual structure, thereby increasing hydrophobicity due to themicro-nano dual structure and the hydrophobic thin film.

In the following, the micro-nano dual structure represents a compositestructure comprising microstructures and nanostructures in whichnanoprotrusions or collapsed nanopores are artificially formed by plasmaetching on the surface of the porous medium with an intrinsicmicrometer-scale roughness.

Since the macroporous substrate has a micrometer-scale surfaceroughness, the micrometer-scale surface protrusions or collapsed poresthereof form the micro-nano dual structure together with theartificially formed nanoprotrusions or nanopores.

Carbon particles of the microporous layer also have a fine surfaceroughness, and thus when the nanostructures such as nanoprotrusions ornanopores are formed on the microporous layer by plasma etching, thenanostructures by the plasma etching form a dual structure on themicroporous layer together with the surface roughness of the carbonparticles.

The surface roughness is given by the nanoprotrusions or collapsednanopores formed on the surface of the material for the porous medium(corresponding to the carbon particles of the microporous layer orcarbon fibers of the macroporous substrate).

As a result, the highly hydrophobic gas diffusion layer with themicro-nano dual structure of the present invention has features that thewettability on both (outer) surfaces is significantly lower than that ofthe conventional gas diffusion layers and the contact angle (i.e.,static contact angle) of a fluid such as pure water on the surface ofthe gas diffusion layer is about 150° or higher, which will be describedin detail later.

Moreover, to overcome the difficulties in achieving high hydrophobicityin the conventional process for forming the gas diffusion layer, highlyhydrophobic properties are imparted to the surface of the gas diffusionlayer (i.e., the surfaces of the microporous layer and the macroporoussubstrate) by both structural and chemical modifications in themanufacturing process of the present invention, in which a process foroptimizing the nanostructures with a high aspect ratio on the surface ofthe gas diffusion layer, a process for structurally modifying thesurface having a micro-nano dual roughness structure, and a chemicalmodification process for forming a chemically hydrophobic surface bydepositing a hydrophobic thin film are performed in combination.

The inventor of the present invention has experimentally found that whendry plasma treatment (i.e., plasma etching) is performed on the gasdiffusion layer, nanoprotrusions or nanopores are firmed (by the plasmaetching) and combined with the surface of the gas diffusion layer with amicrometer-scale surface roughness to form a micro-nano dual structure,that when a hydrophobic carbon thin film is formed on the surface of themicro-nano dual structure by plasma deposition, for example, thehydrophobicity of the gas diffusion layer can significantly increase,and that when the highly hydrophobic gas diffusion layer is used in thefuel cell, the cell performance is improved, and completed the presentinvention. As a result, the highly hydrophobic gas diffusion layer canbe effectively used as the gas diffusion layer for the fuel cell and canefficiently discharge water produced during the electrochemical reactionof the fuel cell.

In the process of modifying the surface of the gas diffusion layer bythe plasma treatment of the present invention, the nanostructures areformed by etching the surface of the gas diffusion layer using argon(Ar) or oxygen (O₂) plasma to provide a structure that can minimize thecontact surface with respect to a fluid such as water, and thehydrophobic thin film (e.g., hydrophobic carbon thin film) is depositedon the surface of the resulting structure. In this case, it is possibleto impart high hydrophobicity or super-hydrophobicity corresponding to acontact angle of about 150° or higher with respect to a fluid such aspure water. That is, only with the dry plasma treatment, the structuraland chemical modifications are possible on the surface of the gasdiffusion layer, and thus it is possible to easily impart highhydrophobicity suitable for the fuel cell.

A better understanding of the increased hydrophobicity on the surface ofthe gas diffusion layer can be achieved by understanding the mechanismof high hydrophobicity or super-hydrophobicity on a solid surface asdescribed below.

The hydrophobicity of the solid surface depends on chemical propertiesof the solid surface, but when a fine pattern is formed on the solidsurface, the hydrophobicity significantly increases such that the solidsurface has super-hydrophobicity. For example, the contact angle of thesurface having a fine protrusion or pore structure with respect to wateris increased to about 150° to 170° to impart super-hydrophobicity,compared to a flat surface which has been subjected to the same chemicaltreatment.

At the same time, the surface having the protrusion or pore structurecan have a self-cleaning function, which allows a droplet on the solidsurface to be readily removed under conditions where the contact anglehysteresis is reduced to less than about 10°. Therefore, in order toform a highly hydrophobic or super-hydrophobic surface, a surface layerhaving low surface energy should be formed and, at the same time, thesurface layer should have a physical/structural surface roughness.

In the case of the surface roughness, the size distribution of fineprotrusions or pores plays a very important role, and the surfaceroughness of collapsed pores also exhibits the same properties as thesurface roughness of fine protrusions. Especially, when the chemicalcomposition of the surface is controlled while the nanometer-scale poresand micrometer-scale pores are present together, a hydrophobic surfaceand, further, a super-hydrophobic surface can be achieved.

Therefore, in the present invention, the target high hydrophobicity isachieved by applying a mechanism for increasing the hydrophobicity,which can be obtained when the above-described physical structure (i.e.,the micro-nano complex structure) and chemical properties are combinedtogether at the surface of the gas diffusion layer.

That is, the highly hydrophobic surface can be obtained by forming anano-roughness pattern by plasma etching and forming a hydrophobic thinfilm by plasma deposition on the surfaces of the microporous layer andthe macroporous substrate, which constitute the gas diffusion layer ofthe fuel cell. Moreover, it is possible to impart high hydrophobicity tothe surface of the gas diffusion layer by simultaneously performingstructural and chemical controls on the surface properties of the gasdiffusion layer.

Moreover, in the exemplary embodiment of the present invention, it ispossible to minimize side effects such as an increase in electricalresistance on the surface of the gas diffusion layer by optimizing theconditions for forming the micro-nano dual structure on the gasdiffusion layer (i.e., plasma etching conditions) and the conditions forhydrophobic thin film coating and to increase the mass transfercharacteristics in the fuel cell and the cell performance by increasingthe surface hydrophobicity.

Next, the present invention will be described in more detail withreference to the drawings. FIG. 1 is a schematic diagram illustrating aprocess of forming a micro-nano complex structure by performing plasmaetching on the surfaces of a gas diffusion layer according to anexemplary embodiment of the present invention. That is, FIG. 1schematically shows a microporous layer and a macroporous substrate,which constitute the gas diffusion layer for the fuel cell, and shows asurface modification method for the gas diffusion layer.

In the fuel cell of the present invention, the highly hydrophobic gasdiffusion layer includes nanostructures with a high aspect ratio and ahydrophobic thin film, which are provided on the surfaces of themicroporous layer and the macroporous substrate.

As shown in FIG. 1, the plasma etching for the micro-nano dual structureis performed on the surface of the microporous layer and the surface ofthe macroporous substrate, respectively, to form nanostructures with ahigh aspect ratio, and a hydrophobic thin film is formed on themicro-nano dual structure by plasma deposition, thereby forming a gasdiffusion layer with a highly hydrophobic surface.

In the present invention, the plasma etching is performed on eachsurface of the gas diffusion layer having the above-described structure,in which the microporous layer and the macroporous substrate arecombined together (see FIG. 1), to form nanoprotrusions or nanopores onthe surface of carbon materials (such as carbon particles and carbonfibers), which constitute the surfaces of the microporous layer and themacroporous substrate (i.e., the surface of the gas diffusion layer).

In an exemplary embodiment, nanoprotrusions or nanopores having a widthof about 1 to 100 nanometers, a length of about 1 to 1,000 nanometers,and thus an aspect ratio of about 1 to 10 may be preferably formed onthe surface of the carbon materials on the surfaces of the microporouslayer and the macroporous substrate by plasma etching.

Here, for example, when the aspect ratio is less than 1, the surfaceroughness effect is not fully established, whereas, when the aspectratio is greater than 10, the structure of the nano-pattern is notstably maintained.

In the microporous layer of the gas diffusion layer, the carbonparticles having non-uniform diameters form aggregates and thus arepresent in the range of several tens of nanometers to severalmicrometers in diameter.

When the plasma etching is performed on the microporous layer, thesurfaces of spherical carbon particles are etched to form sharp carbonparticles having a width of several nanometers, and then the ehydrophobic thin film is deposited on the surface of the resultingmicroporous layer.

Moreover, it is preferred to form nanoprotrusions or nanopores having awidth of about 10 to 30 nanometers, a length of about 10 to 200nanometers, and thus an aspect ratio of 1 to 7 on the surface of carbonfibers having a diameter of about 5 to 20 micrometers of the macroporoussubstrate by plasma etching. These nanoprotrusions form a nano-patternwith a high aspect ratio, thus providing the micro-nano dual structure.

The surface with the micro-nano dual structure has super-hydrophobic andself-cleaning properties.

The hydrophobic thin film for increasing the hydrophobicity may be ahydrocarbon thin film comprising silicon (Si) and oxygen or ahydrocarbon thin film comprising fluorine (F), and the hydrophobic thinfilm may have a thickness in the range of about 0.1 to 90 nanometerssince the thickness of the hydrophobic thin film may have a significanteffect on the performance of the fuel cell.

Here, when the thickness of the hydrophobic thin film is less than 0.1nanometers, the effect of increasing the hydrophobicity of the gasdiffusion layer cannot be obtained, whereas, when it exceeds 90nanometers, the electrical resistance of the gas diffusion layer mayincrease significantly. Therefore, it is preferable that the hydrophobicthin film has a thickness in the range of about 0.1 to 90 nanometers. Inthis thickness range of the hydrophobic thin film, the electricalresistance may be maintained below about 12 mΩcm² at a compressionpressure of about 1 MPa as will be described later.

Especially, for use as the gas diffusion layer of the fuel cell, it isnecessary to properly adjust the thickness of the hydrophobic thin filmso as not to clog the pores without increasing the intrinsic electricalresistance of the gas diffusion layer.

The hydrocarbon material comprising silicon and oxygen may be depositedusing hexamethyldisiloxane (HMDSO) as a precursor, and thehydrophobicity may be controlled by appropriately mixinghexamethyldisiloxane and argon gas (e.g., less than about 30% volumefraction).

The contact angle on the surfaces of the microporous layer and themacroporous substrate in the gas diffusion layer whose hydrophobicity isincreased by the above-described plasma treatment (for the formation ofthe nanostructures by plasma etching and the formation of thehydrophobic thin film by plasma deposition) is about 150° or higher.

Since the nanometer-scale patterns and the micrometer-scale patterns aremixed with PTFE as a hydrophobic polymer on the surfaces of themicroporous layer and the macroporous substrate, which constitute thegas diffusion layer, the contact angle is about 135 to 145°.

However, in the present invention, it is easy to form the microporouslayer and the macroporous substrate having a surface contact angle ofabout 150° or higher by forming a nanostructure surface on the gasdiffusion layer by simple dry plasma treatment and forming a hydrophobicthin film on the surface of the gas diffusion layer by hydrophobic thinfilm coating. Especially, even the surface which is free from PTFE canhave high hydrophobicity corresponding to a contact angle of about 150°or higher.

The reason for this is that the size of the nano-pattern with a highaspect ratio formed by plasma etching is significantly reduced, whichincreases the surface roughness, and the micro-nano dual structure isformed on the surface of the gas diffusion layer. Thus, it is possibleto obtain a contact angle of about 150° or higher, thereby obtaininghighly hydrophobic surface properties. Moreover, the hydrophobic thinfilm is uniformly deposited on the surface such that the surface energyis generally low, and thus a uniform, highly hydrophobic surface can beobtained.

The hydrophobic agent such as PTFE introduced into the microporous layerand the macroporous substrate of the commercially available gasdiffusion layer is difficult to be uniformly introduced into the surfaceand the inside, and a complex wet-forming process should be used.Moreover, it is difficult to increase the contact angle of the surfaceof the gas diffusion layer above about 150°.

The process for forming the above-described highly hydrophobic gasdiffusion layer of the present invention comprises: (a) providing a gasdiffusion layer comprising only a macroporous substrate or both amacroporous substrate and a microporous layer which are combinedtogether; (b) forming nanostructures in the form of nanoprotrusions witha high aspect ratio or collapsed nanopores on the surface of carbonmaterials of the gas diffusion layer; and (c) depositing a hydrophobicthin film on the surface of the gas diffusion layer on which thenanostructures are formed.

As shown in FIG. 1, step (a) is to provide a gas diffusion layercomprising only a macroporous substrate or both a macroporous substrateand a microporous layer which are combined together, and this processfor forming the gas diffusion layer is well known in the art. In anexemplary embodiment, a commercially available gas diffusion layer thatis composed of both the microporous layer and the macroporous substrate,may be used.

Step (b) is to form nanostructures with a high aspect ratio byperforming plasma etching on both sides of the gas diffusion layercomprising the microporous layer and the macroporous substrate on whichthe nanoscale and microscale surfaces are formed. The plasma etching maybe plasma-enhanced chemical vapor deposition (PECVD) or plasma-assistedchemical vapor deposition (PACVD) and may use O₂, Ar, N₂, He, CF⁴, CHF₃,C₂F₆, HF, or SiF₄.

Moreover, in addition to the chemical vapor depositions (CVDs), theetching can be performed by one or a combination of ion beam, hybridplasma chemical vapor deposition (HPCVD), and atmospheric plasma.

When the plasma etching is performed, a large number of nanoprotrusionswith a high aspect ratio are formed, and the size of the nano-patternafter etching is reduced compared to before the etching, which makes thesurface rougher.

The oxygen plasma reacts with the carbon materials to etch the carbonparticles on the surface of the microporous layer and the carbon fiberson the surface of the macroporous substrate. At this time, the carbonmaterials are bonded with oxygen plasma to form CO₂ or CO, and thus thesurface is etched.

In the plasma etching process of the present invention, the size andshape of the nanostructures with a high aspect ratio can be controlledby adjusting at least one of the etching pressure, acceleration voltage,and etching time (i.e., plasma treatment time). Preferably, the etchingpressure is 1 Pa to 10 Pa and the acceleration voltage is −100 Yb to−1,000 Vb.

When the etching pressure is less than 1 Pa, the formation rate of thesurface roughness pattern is too low to effectively form the pattern,whereas, when it exceeds 10 Pa, the formation rate of the surfaceroughness pattern is too high to form a stable pattern.

Moreover, when the acceleration voltage is less than about −100 Vb, theplasma cannot be efficiently generated, whereas, when it exceeds about−1,000 Yb, the plasma generation process cannot be stably maintained.

Further, since the formation of nanostructures on the surface accordingto the plasma etching time may have a significant effect on theperformance of the fuel cell, it is important to perform the plasmairradiation for an optimum time, and thus the plasma etching ispreferably performed for about 0.1 to 60 minutes.

When the plasma etching time is less than about 0.1 minutes, the etchingeffect is too small to allow the nanostructures to develop, whereas,when it exceeds about 60 minutes, it is difficult to control the surfaceshape of desired nanostructures due to excessive etching, and furtherthe cycle of the surface treatment is too long, thus reducing theproductivity.

Step (c) is to deposit a hydrophobic thin film on the surface of the gasdiffusion layer on which a complex pore structure comprising microporesand nanopores is formed. To deposit the hydrophobic thin film, a mixedgas of argon in a partial pressure about 0 to 30 vol % andhexamethyldisiloxane or hexamethyldisiloxane gas may be used.

However, the present invention is not limited to the deposition of thehydrophobic thin film but includes various film deposition methods. Thesurface properties of the hydrophobic carbon thin film for increasingthe hydrophobicity depend on a radio frequency (RF) power supply in aPECVD apparatus and an argon fraction in a precursor gas. Therefore,when the RF power supply and the argon fraction in the precursor gas areproperly controlled, it is possible to control the hydrophobicproperties and achieve an improved thin film.

The gas diffusion layer formed in the above-described manner is stackedwith a membrane-electrode assembly (MEA), a separator with flow fieldsof reactant gas, a gasket for maintaining hydrogen and airtightness,etc. to form a unit cell, and when these unit cells are repeatedlystacked, a polymer electrolyte membrane fuel cell can be manufactured.

The gas diffusion layer is attached to the outer surface of each ofcatalyst layers for the anode and cathode coated on both surfaces of thepolymer electrolyte membrane in the fuel cell and, in the presentinvention, the above-described highly hydrophobic gas diffusion layer isused to manufacture a fuel cell with enhanced mass transfer.

Next, Examples of the present invention will be described in detail withreference to the drawings.

A process of forming a highly hydrophobic gas diffusion layer on thesurfaces of a microporous layer and a macroporous substrate, whichconstitute the gas diffusion layer for a fuel cell, will be described inthe following Examples, but the present invention is not limitedthereto.

EXAMPLES 1. Manufacturing of Gas Diffusion Layers With EnhancedHydrophobicity

First, a commercially available gas diffusion layer material, whichincludes a microporous layer comprising carbon powder and PTFE and amacroporous substrate comprising carbon fibers in the form of felt wasused. While the gas diffusion layer material used had the macroporoussubstrate without PTFE hydrophobic treatment and the microporous layerwith PTFE hydrophobic treatment, a commercially available gas diffusionlayer material having a macroporous substrate comprising carbon fibersin the form of Ht and PTFE may be applied to the present invention. Thediameter of carbon particles forming the microporous layer in the gasdiffusion layer material is not uniform and lies in the range of 10 to300 nanometers, and the carbon fibers forming the macroporous substrateare present in the range of 5 to 20 micrometers in diameter.

The surfaces of the microporous layer and the macroporous substrate ofthe prepared gas diffusion layer material were subjected to oxygenplasma etching using RF-PECVD, and the oxygen plasma etching wasperformed under conditions where only oxygen was used as gas, theetching pressure was 1 to 10 Pa, and the RF voltage was −100 Vb to−1,000 Vb.

The oxygen plasma reacts with the carbon materials to etch the carbonmaterial on the surfaces of the microporous layer and the macroporoussubstrate and, at this time, the carbon materials are bonded with oxygenplasma to form CO₂ or CO, thus etching the surfaces of the microporouslayer and the macroporous substrate.

Then, HMDSO comprising silicon and oxygen was coated on the surface ofthe carbon nanoprotrusions formed by the plasma etching to increase thehydrophobicity, thus forming a hydrophobic thin film.

The hydrophobic thin film was deposited using HMDSO by 13.56 MHzRF-PECVD on the surfaces of the microporous layer and the macroporoussubstrate, in which the fraction of argon gas in the precursor gas wasmaintained at 0 vol % and the RF power source was fixed to −400 Vb.

At this time, the thickness of the hydrophobic thin film might have asignificant effect on the performance of the fuel cell.

As an example, when the surfaces of the microporous layer and themacroporous substrate etched by oxygen plasma are deposited with theHMDSO hydrophobic thin film for more than 1 minute, the thickness of theHMDSO hydrophobic thin film is increased to about 100 nanometers orhigher. In this case, the gas diffusion layer has an electricalresistance of about 50 mΩ cm² at a compression pressure of 1 MPa duringa first compression, compared to the typical electrical resistance ofabout 12 mΩ cm². Moreover, the cell voltage at 1,500 mA/cm² issignificantly reduced by about 10% or higher in a normal operatingcondition where the stoichiometric ratio (anode/cathode) is 1.5/2.0.

As another example, when the surfaces of the microporous layer and themacroporous substrate etched by argon plasma are deposited with theHMDSO hydrophobic thin film for more than 1 minute, the thickness of theHMDSO hydrophobic thin film is increased to about 100 nanometers orhigher. In this case, the electrical resistance of the gas diffusionlayer is significantly increased to about 126 mΩ cm², which is about 10times the typical electrical resistance of about 12 mΩ cm² at acompression pressure of 1 MPa during the first compression. Moreover,the cell voltage at 1,500 mA/cm² is significantly reduced by about 70%or higher in a normal operating condition where the stoichiometric ratio(anode/cathode) is 1.5/2.0, thus causing a catastrophic cell failurewhere the fuel cell does not operate any longer.

Therefore, in this Example, to provide an optimized thickness of thehydrophobic thin film, with which the hydrophobicity of the gasdiffusion layer could increase and the increase in electrical resistancecould be minimized, the HMDSO coating time was controlled to 10 secondssuch that the hydrophobic thin film had a uniform thickness of about 10nanometers at a chamber pressure of 5 Pa.

FIG. 2 is scanning electron microscope (SEM) images of the surfaces ofgas diffusion layers (i.e., microporous layers and macroporoussubstrates) before and after oxygen plasma etching for forming amicro-nano dual structure on the surfaces of gas diffusion layers andafter “oxygen plasma etching+hydrophobic carbon thin film (HMDSO)coating”.

(a) is an SEM image at 30,000× magnification of a microporous layer of agas diffusion layer material before surface treatment, and (b) and (c)are SEM images of microporous layers after surface treatment (etched byoxygen plasma for 5 minutes and coated with hydrophobic HMDSO for 10seconds) in the Example of the present invention. Here, (b) is an imageat 30,000× magnification, and (c) is an image at 100,000× magnification.

Moreover, (d) is an SEM image of a macroporous substrate of a gasdiffusion layer material before surface treatment, and (e) and (f) areSEM images of macroporous substrates after surface treatment (etched byoxygen plasma for 5 minutes and coated with hydrophobic HMDSO for 10seconds) in the Example of the present invention. Here, (e) is an imageat 25,000× magnification, and (f) is an image at 100,000× magnification.

Referring to the SEM images of the gas diffusion layers before thesurface treatment, the carbon particles of the microporous layer havenon-uniform diameters and are agglomerated to form aggregates before thesurface treatment. Thus, the carbon particles are present in the rangeof several tens of nanometers to several micrometers in diameter (see(a) of FIG. 2).

On the contrary, referring to the SEM images of the gas diffusion layersafter the surface treatment, it can be seen that the diameter of thecarbon particles is reduced to 10 to 50 nanometers to formnanoprotrusions, thus forming a micro-nano dual structure with a furtherroughened surface (see (b) and (c) of FIG. 2).

Moreover, when the surface of the macroporous substrate is etched byoxygen plasma for 5 minutes and coated with hydrophobic HMDSO for 10seconds (see (e) and (f) of FIG. 2), compared to before the surfacetreatment (see (d) of FIG. 2), a roughness structure such asnanoprotrusions is formed on the surface of the carbon fibers. Here, thenanoprotrusions have a width of 10 to 30 nanometers and a length of 10to 200 nanometers, providing an aspect ratio of about 1 to 7. Therefore,the carbon fibers having a diameter of 5 to 20 micrometers and thenanoprotrusions with a high aspect ratio formed thereon form a surfacewith a micro-nano dual structure. As a result, the surface with amicro-nano dual structure can be formed, and thus a surface structurehaving super-hydrophobic and self-cleaning properties can be completed.

2. Measurement of Contact Angles of Gas Diffusion Layers

The contact angle of the gas diffusion layer in the Example formed inthe above manner was measured, and that of the gas diffusion layermaterial before surface treatment in a Comparative Example was alsomeasured.

Moreover, to examine the change in contact angle of the gas diffusionlayers according to oxygen plasma etching and HMDSO coating conditions,the contact angle of the gas diffusion layer in an additional Exampleetched by oxygen plasma for 5 minutes and coated with hydrophobic HMDSOfor 5 or 10 seconds was also measured.

The measurement of the contact angle was performed using a goniometer(Data Physics Instrument GmbH, OCA 20 L). This instrument makes itpossible to acquire the optical image and contact angle of a sessiledroplet on the surface. The contact angle (i.e., static contact angle)was measured by gently landing a 5 μl droplet on the surface. The“contact angle” used for convenience in the present invention means the“static contact angle”.

FIG. 3 shows the changes in contact angle of the microporous layers andthe macroporous substrates of the gas diffusion layers according tooxygen plasma etching and HMDSO coating conditions.

In the case of the gas diffusion layer without the surface treatment inthe Comparative Example, the intrinsic contact angles of the microporouslayer and the macroporous substrate are 145° and 121°, respectively.

However, in the case of the gas diffusion layer, whose microporous layerand macroporous substrate were all subjected to the surface treatmentaccording to the Example of the present invention, the contact angle wassignificantly increased to 150° or higher, from which it can be seenthat the super-hydrophobicity was exhibited.

That is, in the case of the surface treatment of oxygen plasma etchingfor 5 minutes and HMDSO coating for 5 seconds, the contact angles of themicroporous layer and the macroporous substrate were increased to 162°and 156°, respectively, and in the case of the surface treatment ofoxygen plasma etching for 5 minutes and HMDSO coating for 10 seconds,the contact angles of the microporous layer and the macroporoussubstrate were increased to 164° and 155°, respectively.

3. Measurement of Electrical Resistances of Gas Diffusion Layers

The electrical resistances of the gas diffusion layers in theComparative Example (before surface treatment) and the Example (aftersurface treatment) according to the compression pressure were measured.At this time, a commercially available tester (Model: CPRT Tester, LCDVCo., Korea) was used to measure the changes in electrical resistance ofall gas diffusion layer samples according to the compression pressure infirst and second compression states.

FIG. 4 is a graph illustrating the changes in electrical resistance ofgas diffusion layer samples according to the time of oxygen plasmaetching for forming a micro-nano dual structure and hydrophobic carbonthin film (HMDSO) coating conditions during the first compression. It isshown that the electrical resistance of the gas diffusion layer withoutthe surface treatment in the Comparative Example is dramaticallydecreased with increasing the compression pressure. In the case of thegas diffusion layers in which the surfaces of the microporous layer andthe macroporous substrate are all treated (i.e., oxygen plasma etchingfor 5 minutes+HMDSO coating for 5 seconds & oxygen plasma etching for 5minutes+HMDSO coating for 10 seconds) in the Example, it is alsoobserved that the electrical resistances are significantly decreasedwith increasing the compression pressure, which are similar to that ofthe gas diffusion layer without the surface treatment in the ComparativeExample.

FIG. 5 is a graph illustrating the changes in electrical resistance ofgas diffusion layer samples according to the time of oxygen plasmaetching for forming a micro-nano dual structure and hydrophobic carbonthin film (HMDSO) coating conditions during the second compression. Incomparison with the results of the first compression in FIG. 4, it canbe seen that the electrical resistance of the gas diffusion layerwithout the surface treatment in the Comparative Example is small evenat the initial compression pressure of 0.03 MPa and then is reduced withincreasing the compression pressure. In the case of the gas diffusionlayers in which the surfaces of the microporous layer and themacroporous substrate are all treated (i.e., oxygen plasma etching for 5minutes+HMDSO coating for 5 seconds & oxygen plasma etching for 5minutes+HMDSO coating for 10 seconds) in the Example, it is alsoobserved that the electrical resistances are small even at the initialcompression pressure of 0.03 MPa and then is reduced with increasing thecompression pressure, which are very similar to that of the gasdiffusion layer without the surface treatment the Comparative Example.

The above measurement results of the electrical resistance are shown tobe suitable for manufacturing the gas diffusion layer for the fuel cellsince the oxygen plasma etching and HMDSO coating conditions of theExample can increase the surface contact angle, which increases thehydrophobicity, and can maintain the electrical resistance of the gasdiffusion layer below 12 mΩ cm² at a compression pressure of 1 MPa orhigher, at which the unit cells are typically connected to each other.

4. Measurement of Fuel Cell Performance

Fuel cells were prepared using the gas diffusion layers according to theExample of the present invention and the Comparative Example and thenthe electrochemical performances were evaluated. That is, the fuel cellswere prepared by attaching the gas diffusion layers according to theExample and the Comparative Example to the outer surfaces of thecatalyst layers for the anode and cathode coated on both surfaces of themembrane-electrode assembly, and their performance was evaluated.

During the evaluation of the electrochemical performance of the fuelcells, current-voltage polarization characteristics of 25 cm² singlefuel cells were measured and compared using a commercially availabletester (Won-A Tech Co., Ltd., Korea).

At this time, the electrochemical performance of the fuel cell stackswith the gas diffusion layers according to the Example and theComparative Example were evaluated under the following conditions:

Temperature at fuel cell inlet: 65° C.

Relative humidity of Hydrogen(anode)/Air (cathode): 100%/100%

Stoichiometric Ratio of Hydrogen (anode)/Air (cathode): 1.5/2.0 or1.1/1.5

FIG. 6 shows the changes in fuel cell performance before and afteroxygen plasma etching and HMDSO coating in a condition where thestoichiometric ratio of reactant gases (anode/cathode) is 1.5/2.0.

The results of FIG. 6 shows the fuel cell performances measured in acondition where the stoichiometric ratio of reactant gases(anode/cathode) was 1.5/2.0, which is widely applied in a normaloperating condition of a fuel cell vehicle.

It can be seen that, in all the current density range measured, thevoltage of the fuel cell employing the gas diffusion layer (after thesurface treatment such as oxygen plasma etching for 5 minutes+HMDSOcoating for 10 seconds) in the Example is higher than that of the fuelcell employing the gas diffusion layer without the surface treatment inthe Comparative Example.

As an example, at a high current density of 1,500 mA/cm², the cellvoltages of the Comparative Example and the Example are 0.507 V and0.534 V, respectively, from which it can be seen that the cellperformance of the Example of the present invention is higher, about 5%.

FIG. 7 shows the changes in fuel cell performance before and afteroxygen plasma etching and HMDSO coating in a condition where thestoichiometric ratio of reactant gases (anode/cathode) is 1.1/1.5.

The results of FIG. 7 are an example of evaluating the performancestability of the fuel cell in an abnormal operating condition of thefuel cell vehicle and measured in a condition where the stoichiometricratio of reactant gases (anode/cathode) was 1.1/1.5, where the reactantgases such as hydrogen and air (oxygen) are not sufficiently supplied byunexpected external factors.

As such, it is very important to develop a fuel cell, in which theperformance is maintained at its maximum and is not significantlyreduced during reactant gas starvation, for the purpose of increasingthe driving stability of the fuel cell vehicle.

Comparing the results of FIG. 7 with those of FIG. 6, the cell voltageswere all reduced in the Comparative Example and the Example due to thelow stoichiometric ratio of reactant gases. However, it can be seenthat, in all the current density range measured, the voltage of the fuelcell employing the gas diffusion layer (after the surface treatment suchas oxygen plasma etching for 5 minutes+HMDSO coating for 10 seconds) inthe Example is higher than that of the fuel cell employing the gasdiffusion layer without the surface treatment in the ComparativeExample.

In particular, it can be seen that the difference increases at a highcurrent density of 1,500 mA/cm² or higher. As an example, at a highcurrent density of 1,500 mA/cm², the cell voltages of the ComparativeExample and the Example are 0.365 V and 0.437 V, respectively, fromwhich it can be seen that the cell performance of the Example of thepresent invention is higher, about 20%, and the performance of the fuelcell in the Example is relatively more stable.

In summary, it can be seen from the measurement results of the gasdiffusion layer characteristics and the fuel cell performance that, dueto the oxygen plasma etching and hydrophobic HMDSO thin film coating onthe surface of the gas diffusion layer of the Example of the presentinvention, the hydrophobicity of the gas diffusion layer increases, theincrease in electrical resistance is minimized, and the mass transfercharacteristics are enhanced, thus exhibiting excellent cellperformance.

As such, according to the manufacturing process of the presentinvention, it is possible to significantly improve the hydrophobicproperties of the surfaces of the microporous layer and the macroporoussubstrate and to manufacture a highly hydrophobic gas diffusion layer.

Especially, when the process of improving the hydrophobic properties ofthe surfaces of the microporous layer and the macroporous substrateaccording to the present invention is used, it is possible tosignificantly improve the hydrophobic properties of the gas diffusionlayer formed by the microporous layer and the macroporous substrate.

Moreover, when the above-described highly hydrophobic gas diffusionlayer is used, it is possible to manufacture a fuel cell with enhancedmass transfer characteristics.

As described above, according to the fuel cell and the method formanufacturing the same in accordance with the present invention, it ispossible to provide a fuel cell with excellent mass transfercharacteristics using the gas diffusion layer that exhibits highhydrophobicity due to the nanostructures with a high aspect ratio andthe hydrophobic thin film formed on the surfaces of the microporouslayer and the macroporous substrate.

In particular, the contact angle significantly increases on the surfaceof the highly hydrophobic gas diffusion layer used in the fuel cell ofthe present invention, thus exhibiting super-hydrophobic surfaceproperties. This super-hydrophobic surface has a self-cleaning function,which ejects a droplet from the surface. Thus, the fuel cell withenhanced mass transfer characteristics according to the presentinvention can efficiently discharge water produced during theelectrochemical reaction, improving the overall cell performance.

Most of all, it is possible to improve the cell performance in a highpower density region and in an abnormal operating condition, and thus itis possible to stably maintain the cell performance under abnormaloperating conditions such as rapid acceleration/high speed driving ofthe fuel cell vehicle or during reactant gas starvation.

The invention has been described in detail with reference to preferredembodiments thereof. However, it will be appreciated by those skilled inthe art that changes may be made in these embodiments without departingfrom the principles and spirit of the invention, the scope of which isdefined in the appended claims and their equivalents.

1. A fuel cell with enhanced mass transfer characteristics, the fuelcell comprising: a membrane-electrode assembly, a gas diffusion layer asa porous medium, and a separator having flow fields of reactant gases,which are stacked to form a unit cell, wherein the gas diffusion layerincludes a micro-nano dual structure, in which nanometer-scaleprotrusions or collapsed pores are formed on the surface of the porousmedium with a micrometer-scale surface roughness, and a hydrophobic thinfilm is deposited on the surface of the micro-nano dual structure. 2.The fuel cell of claim 1, wherein the gas diffusion layer is a porousmedium comprising only a macroporous substrate or both a macroporoussubstrate and a microporous layer coated on the macroporous substrate.3. The fuel cell of claim 2, wherein the macroporous substrate comprisesa micro-nano dual structure in which nanoprotrusions or nanopores havinga width of 10 to 30 nanometers, a length of 10 to 200 nanometers, and anaspect ratio of 1 to 7 are formed on the surface of carbon fibers havinga diameter of 5 to 20 micrometers.
 4. The fuel cell of claim 1, whereinthe nanoprotrusions or nanopores have a width of 1 to 100 nanometers, alength of 1 to 1,000 nanometers, and an aspect ratio of 1 to
 10. 5. Thefuel cell of claim 1, wherein the hydrophobic thin film is a hydrocarbonthin film.
 6. The fuel cell of claim 5, wherein the hydrophobic thinfilm is a hydrocarbon thin film comprising silicon and oxygen or ahydrocarbon thin film comprising fluorine.
 7. The fuel cell of claim 1,wherein the thickness of the hydrophobic thin film is set such that thegas diffusion layer has an electrical resistance of 12 mΩ cm² or lowerat a compression pressure of 1 MPa.
 8. The fuel cell of claim 1, whereinthe hydrophobic thin film has a thickness of 0.1 to 90 nanometers. 9.The fuel cell of claim 1, wherein the surface on which the hydrophobicthin film is formed has a static contact angle of 150° or higher withrespect to pure water. 10-22. (canceled)